Mk3 Rear brake Problems

Bilko said:

Wow, I had no idea that vacuum bleeding cause these issues.

I'm going to try and bleed the system the conventional way and see if I can get a better result.

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I quit using my vacuum bleeder on ANY vehicle 15 years ago.
I adapt my pressure bleeder (most recently on a YZ250 that was stubborn) to motorcycles, no problem.
Not a lot of money.

marshg246 said:

1) Screw in a bolt (5/16" UNF if I remember right) in the feed line hole to seal it

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3/8" UNF.

Braided lines can often be over-length. If the line is higher than the caliper as below then it can be tied to the swingarm to remove the chance of air being trapped at the top of the loop.

Thanks for all the great tips guys, this community is a great resource for information and ideas.

I was reluctant to post about my problem, but the response has been great with some really useful info. Plus it will be here for others searching for help in the future.

I'm gonna take some notes and get back in that garage!

Could the issue of only getting one piston moving be due to having the stuck one fully pushed home in its bore, thus effectively sealing off any fluid volume behind it? I recall hearing not to push it fully home when initially setting the caliper up (at least for the front one).

Tornado said:

Could the issue of only getting one piston moving be due to having the stuck one fully pushed home in its bore, thus effectively sealing off any fluid volume behind it? I recall hearing not to push it fully home when initially setting the caliper up (at least for the front one).

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That's the inner piston. Same for front or rear (calipers are the same).

If the outer pad is against the disc and the piston is against the pad then the outer piston should be clear of the drilling.

Tornado said:

Could the issue of only getting one piston moving be due to having the stuck one fully pushed home in its bore, thus effectively sealing off any fluid volume behind it? I recall hearing not to push it fully home when initially setting the caliper up (at least for the front one).

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When I stripped it down (for the 3rd time) and removed the end plug there was fluid in the front bore so it was getting some in there.

Pressing the brake pedal was causing the inner piston to move the disc a small amount, so at least one side had pressure, but when I took it off and got it on the bench, both pistons were moving freely.

L.A.B. said:

3/8" UNF.

Braided lines can often be over-length. If the line is higher than the caliper as below then it can be tied to the swingarm to remove the chance of air being trapped at the top of the loop.

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My braided ( as supplied ) sits high , but not as high as pictured .
Without reviewing the entire thread , get rid of rubber line and go braided , as the rubber line may have a micro-leak ?

So with everything back together, re-using the AN seals but bleeding manually (no vacuum pump), I got decent pressure at the pedal with pads biting together and releasing.

I've been out on a couple of test rides, using the brakes as normal and everything seems fine. Disc is getting hot but no more than I would expect with new pads.

I think the master cylinder was to blame as the barrel took a few turns out from where it was factory-set to get it right. As said by someone earlier, new doesn't mean right.

Thanks everyone for all the help!

On cars its always the pushrod that passes through the booster to the master cyl , when there is no free board the brakes pump up , so make sure the link between the pedal and the master piston has got some clearance .

Shelby-Right said:

when there is no free board the brakes pump up , so make sure the link between the pedal and the master piston has got some clearance .

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The pedal linkage* is rigidly attached to the master cylinder pushrod, in fact, the master cylinder assembly must be screwed into the casting until all play is just removed from the pushrod, then one turn plus enough to align the groove with the grub screw.

*(Can, however, be adjusted to raise/lower the pedal height)

Last week I had my rear brake lock up. It destroyed the Ferodo pads and bubbled paint on the rotor. Turns out the problem was the new Girling MC I installed last year. Like the OP, I assumed it would be adjusted correctly from the manufacturer... it wasn't. When I attempted to unscrew the cylinder, it wouldn't move more than a quarter turn. When they assembled it, the set screw wasn't lined up with the machined slot in the cylinder when it was tightened, and it ended up mangling the threads. Suspect this was partially due to the slot cut such that the stud won't line up with the MC body to allow the reservoir to be perfectly upright... about 5 degrees inboard. Using oil and twisting the cylinder in and out repeatedly allowed me to get it free. Turns out it was multiple full turns too far in, no idea how it worked at all. Once set correctly, the air test showed lots of bubbles whereas before an occasional bubble would float up. Lots to unpack there, but safe to say quality control has taken a holiday at Girling.

I went for a long ride yesterday in the mountains without any problems so I'm assuming its fixed. However, the rear caliper still gets pretty warm whereas the front doesn't. Nothing like when it was locked up, but still warm. The pads are dragging and showing some resistance when I spin the wheel by hand, so the pistons aren't retracting all the way, but I believe all my bikes do that to some degree. I rebuilt the caliper when I adjusted the MC and didn't see a problem. Wondering if the new pads need to bed in before this resolves itself, or perhaps it's normal, time will tell.